The present invention relates to a flexographic rotary platen printing press utilized for printing indicia onto a flexible web, and, more particularly, to a flexographic rotary platen press having a printing system that makes the printing process easier and with accurate indexing of multiple printing colors.
The use of various printing presses are, of course, well know for their use in providing the printing of a web of flexible material, such as plastic material used in packaging industry for various products. In such systems for printing and in general, the packaging machine, or host machine, actually pulls the web through the print station and that host machine therefore normally provides the movement of the web as well as the registration to insure that the indicia is printed on to the web at the proper location along that web and that each of that indicia is properly spaced along the web.
One typical system currently used for such purpose is the platen press where the web is positioned on the platen and held motionless while a generally planar printing plate is impressed against that portion of the web, sandwiching the web material between the printing plate and the platen. While that type of system allows good registration of the indicia to be printed onto the web, there is an inherent fault in that it is extremely difficult to maintain an even print over such as large area, that is, the printing plate must be precisely planar and parallel to the plane of the platen or the eventual printed indicia is uneven, and exhibits a non-uniform pattern.
Another current system is with the use of a flexographic press where the web is continually in motion and is moved along its longitudinal axis by the various mechanisms of the host machine. In that system, there is a rotary print drum that is rotated and the web is moved intermediate that rotating printing drum and a round impression roller as opposed to the flat platen of the previous platen press. The cylindrical rotary print drum has the desired indicia on a printing plate located on the external peripheral surface of that printing drum and rotates in the same direction as the web. An inking system is used to apply the ink to the printing roller and that inking system is well know and uses a anilox roller to apply the ink to the particular indicia on the print drum.
Again, however, the aforedescribed system has certain limitations as the moving web must be carefully aligned with the platen and the printing roller and thus is carried through a large number of idler rollers that guide the web to the proper position for printing, including an impression roller that requires the web to form a complete S configuration for correct alignment, registration and for providing the proper tension at all times on the moving web.
Thus, with the moving web type of printing press, the system requires a large number of tensioning and idler rollers to hold the web in the correct alignment and to create the proper tension in the web as it moves in a serpentine fashion through the flexographic printing press. Each roller, therefore, changes the direction of the web as it is moving through the printing press and each roller and change of direction also induces a stress on the web and can cause a stretching of the web and result in the final printed indicia being blurred. Accordingly, while the use of a multiplicity of tensioning and other rollers is necessary in the operation of the aforedescribed press to carry out printing on the fly, there is also a downside, that is, while the rollers are essential in the overall control of the web tension and control of the web orientation, the use of so many rollers generally creates many changes indirection of the web and also introduces stresses in the web that are undesirable.
The problem is aggravated with the use of multicolor printing where different colors are used since those different colors are printed on to the web serially, that is, after the printing of one color on to the web, the web, still moving at a high speed, progresses to a further printing cycle where the next color is applied to the web and so on. Obviously then, with a multi color printing, there are two, three of additional sets of rollers through which the web must travel and, again, each additional set of rollers can cause stress, stretch the web and make the registration of the subsequent colors extremely difficult. Too, with the multiple color printing presses, there is no room to insert dryers intermediate the color printing steps and thus the system is limited to materials having some porosity of the web material to allow the ink to dry before passing through a later printing stage with a different color. Accordingly, the use of such a multi-color rotating drum printing press is inappropriate for the other non-porous materials of webs, such as polyethylene, nylon, or polypropylene.
Thus, it would be advantageous to provide a flexographic printing press that can print the desired indicia upon a web without an excess of tensioning rollers or S-bends, be able to be used on non-porous materials and yet achieve a good printing quality and accuracy of the position and registry of the printed material.
The present invention provides an improved flexographic rotary platen printing press and system used by that printing press to print indicia upon a web. With the present printing system, there is provided a system that operates in conjunction with a host machine that advances or moves the web in an intermittent manner, that is, the host machine preferably advances the web along its longitudinal axis and stops the web at predetermined intervals where the actual printing of the web is carried out. The overall movement of the web itself can be carried out by a conventional means including a D.C. or stepper motor in the host machine and controlled to move and stop the progress of the web at a precise location where it is desired to place the printed indicia. Alternatively, the present flexographic rotary platen printing system can be used with some independent moving system that can control the movement of the web and not necessarily be a part of a host machine that is carrying out some other operation on the web material.
With the present invention, however, as will be seen, the actual speed of the advancement of the web is not critical and need not be subject to any particularly precise electronic control since that speed does not need to be coordinated with the speed of any print drum; only the position of the web in the stopped position is of importance.
Thus, the web is stopped in the precise desired position and a print drum, having the print plate carried thereon, is activated and moves transversely across the web as the print drum also rotates. As the print drum moves along the transverse direction, the web is sandwiched between the print drum and a flat platen, such that the printing of the web takes place. When the print drum reaches the end of its travel across the width of the web, the printing is completed and the print drum is returned to its original location to be ready for a subsequent cycle.
As is known, the inking process can also be carried out by conventional inking systems including the use of an anilox roller and the inking system may utilize a closed doctor blade type utilizing a laser engraved ceramic coated anilox roller mounted in a stationary position off to the side of the web and fed with ink by an inverted bottle cartridge of pigmented ink. In the inking system, the anilox roller is driven, preferably by a stepper motor, and that stepper motor is coordinated with a stepper motor that drives the print drum during the inking process such that the two motors are synchronized electronically to assure that the surface speeds of the anilox roller and the print plate on the print drum match without the need to link the two motors or drives together with gears. As will be noted, the various rotating motive means will be described herein with the preferred means, that is by use of stepper motors, however, it will be seen that other motive means, including servo motors, could be used in carrying out the present invention and still be within the inventive concept. Of importance, however, is that with the use of speed controllable motors, the coordination of the speeds between the anilox roller and the print drum can be carried out electronically and thus, without the need for any gearing system to provide that coordination of such speeds.
Thus, the print drum is also driven by a stepper motor and the motor speed can be controlled as the print drum moves transversely across the web in carrying out the printing process. A further stepper motor operates the linear drive system utilized to move the print drum in the transverse direction and that linear drive system can comprise mounting the print drum, its bearing housing and its stepper motor on a trolley that slides along a pair of rails. A belt or sprocketed chain can connect the linear drive motor to the trolley and by operation of the linear drive motor, the trolley carrying the print drum, the print motor and the print drum bearing housing bi-directionally transverse to the movement of the web, when in motion.
Taking, therefore, a complete cycle for a single color print, the print drum will make one complete revolution to rotate the print plate against the anilox roller to ink the print plate. At this step, the stepper motor rotating the anilox roller and the stepper motor rotating the print drum are electronically controlled to match the speed of the anilox roller with that of the print plate. Once inked, the print drum will disengage from the ink system and move transversely across the web while simultaneously being rotated by its stepper motor.
At this step, the speed of the rotation of the print drum is controlled to be coordinated with the linear speed of the print drum as it moves transversely across the web to print the desired indicia on the web at a precise registration. The web, being sandwiched between the print drum and a flat platen, is therefore at a standstill and the printing process can be carried out accurately and the contact between the printing plate and the flat platen is a line contact. The print drum continues its progress, moved by the linear drive motor, from the first or home position fully across the web to a remote position past the web where its motion is terminated and printing has been accomplished. Upon that completion of the print step, the print drum will lift away from the platen and return to the home position juxtaposed to the inking system ready to carry out the process again.
As such, the entire process for a print cycle can be carried out by means of three stepper motors; an anilox motor to rotate the anilox roller, a linear drive motor to move the print drum in a linear path across the web and a print drum motor that rotates the print drum. All coordination of the motors can be readily be accomplished electronically by a microprocessor based system.
Other features of the present flexographic rotary platen printing press will become apparent in light of the following detailed description of a preferred embodiment thereof and as illustrated in the accompanying drawings.